A BEGIN rule is executed once only, before the first input record
is read. Likewise, an END rule is executed once only, after all the
input is read. For example:
$ awk '
> BEGIN { print "Analysis of \"li\"" }
> /li/ { ++n }
> END { print "\"li\" appears in", n, "records." }' mail-list
-| Analysis of "li"
-| "li" appears in 4 records.
This program finds the number of records in the input file mail-list
that contain the string ‘li’. The BEGIN rule prints a title
for the report. There is no need to use the BEGIN rule to
initialize the counter n to zero, as awk does this
automatically (see Variables).
The second rule increments the variable n every time a
record containing the pattern ‘li’ is read. The END rule
prints the value of n at the end of the run.
The special patterns BEGIN and END cannot be used in ranges
or with Boolean operators (indeed, they cannot be used with any operators).
An awk program may have multiple BEGIN and/or END
rules. They are executed in the order in which they appear: all the BEGIN
rules at startup and all the END rules at termination.
BEGIN and END rules may be intermixed with other rules.
This feature was added in the 1987 version of awk and is included
in the POSIX standard.
The original (1978) version of awk
required the BEGIN rule to be placed at the beginning of the
program, the END rule to be placed at the end, and only allowed one of
each.
This is no longer required, but it is a good idea to follow this template
in terms of program organization and readability.
Multiple BEGIN and END rules are useful for writing
library functions, because each library file can have its own BEGIN and/or
END rule to do its own initialization and/or cleanup.
The order in which library functions are named on the command line
controls the order in which their BEGIN and END rules are
executed. Therefore, you have to be careful when writing such rules in
library files so that the order in which they are executed doesn’t matter.
See Command-Line Options for more information on
using library functions.
See A Library of awk Functions,
for a number of useful library functions.
If an awk program has only BEGIN rules and no
other rules, then the program exits after the BEGIN rules are
run.41 However, if an
END rule exists, then the input is read, even if there are
no other rules in the program. This is necessary in case the END
rule checks the FNR and NR variables, or the fields.
The original version of awk kept
reading and ignoring input until the end of the file was seen.